1. Origin of the Invention
The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 U.S.C. 202) in which the Contractor has elected not to retain title.
2. Field of the Invention
This invention relates to dilute magnetic semiconductors, and more particularly, to preparation of a semiconductor film of the CdTe-MnTe alloy system by metalorganic chemical vapor deposition.
3. Description of the Relevant Art
Dilute magnetic semiconductors (DMS) are pseudobinary Group II-Group VI semiconductors in which a fraction x of the Group II elemental sites are occupied by a transition metal magnetic ion, usually Mn. The CdTe-MnTe alloy system is perhaps the most extensively studied member of this class of materials. Single phase solid solutions of this II-VI semiconductor with zincblende structure are formed in the system Cd.sub.1-x Mn.sub.x Te, wherein 0.ltoreq..times..ltoreq.0.7. The energy gap at room temperature (E.sub.g =1.50+1.34.times.eV) and lattice constant (a.sub.o= 6.487-0.149.times..ANG.) vary linearly throughout this range, even though MnTe has a different structure (nickel arsenide) from that of CdTe (zincblende). The presence of the paramagnetic manganese ions in the host lattice (CdTe) gives rise to interesting magnetic and magneto-optical properties because of spin-exchange interactions with conduction band electrons and valence band holes which modify the band structure. Such interesting phenomena include a giant Faraday rotation near the fundamental absorption edge, an extremely large negative magneto-resistance, Stokes-shifted spinflip Raman scattering, and very large electronic g-factors.
Heteroepitaxial thin films of (CdMn)Te have heretofore been deposited by molecular beam epitaxy (MBE), primarily on GaAs substrates. Subsequently, multiple quantum well and superlattice structures also have been prepared. These structures have exhibited strong photoluminescence, stimulated emission and magnetically tuneable lasing action. It is well known, however, that MBE deposition is not the method of choice for commercial-scale preparation of components for photovoltaic and magneto-optical devices, uses for which a (CdMn)Te or similar DMS would otherwise be well-suited on account of their properties. For example, (ZnMn)Se is of particular interest, as its bandgap lies in the visible wavelength range. At the very least, MBE deposition considerably increases unit production costs for products relative to those prepared by metalorganic chemical vapor deposition (MOCVD) techniques. As compared with MBE deposition, MOCVD permits higher through-put sample growth and allows for deposition of a film over a larger, more uniform area. Thus, MOCVD techniques, if applicable, would significantly reduce costs and simplify manufacture.
Growth of (CdMn)Te has also been accomplished by ionized cluster beam deposition (T. Koyanagi et al., J.Appl.Phys. 61, 3020 (1987)). As is the case with MBE deposition, however, such a process would also not be practical for commercial applications. In particular, ionized cluster beam deposition methods require the use of high vacuums, although not as high as with MBE methods. Moreover, deposition rates are generally low, and there are problems with deposition area and uniformity.
There has been to date no report of (CdMn)Te deposition by MOCVD techniques, although high-quality heteroepitaxial CdTe films have been grown on GaAs, InSb and sapphire substrates. One practical reason for this has probably been the lack of a reasonably high vapor pressure metal alkyl source for Mn.